Soil Perc Testing

What a Soil Test Does

A soil test determines the site and soil suitability for a Private Onsite Wastewater Treatment System (POWTS). It guides the type, size, location, and design of new and replacement systems. Not every replacement must be a mound system. The correct system depends on the actual soil profile and limiting factors observed during the evaluation.  More Information can be found here (Wisconsin State Legislature SPS 385)

Soil Conditions Vary By Lot

Neighboring parcels can differ dramatically. Some sites qualify for conventional subsurface systems. Others require mound systems or alternative designs that meet code and protect groundwater.

Soil and Site Evaluation vs “Perc Test”

Wisconsin uses a Soil and Site Evaluation performed by a Certified Soil Tester. This is often called a “soil test” and is different from the old-style “perc test.” Traditional percolation tests only measured how fast water infiltrates and did not reveal how far water travels before hitting bedrock or groundwater. Modern evaluations document horizons, textures, structure, mottling/redoximorphic indicators, depths, and other features to design a code-compliant system. More Information can be found here (Wisconsin State Legislature SPS 385)

Limiting Factors You Need To Know

A limiting factor is typically high bedrock or high groundwater. Even with a “fast” perc rate, if the wastewater quickly encounters a limiting factor, it will not be treated properly and can contaminate groundwater or cause premature system failure. Wisconsin code requires a minimum three feet of unsaturated soil between the infiltrative surface and a limiting condition for most systems. More information can be found in the Wisconsin private onsite wastewater treatment system (powts) inspector’s handbook.

Why Mound Systems Are Sometimes Required

If the evaluation finds groundwater or bedrock too close to grade, a mound system raises the treatment zone to maintain the three-foot separation and provide proper treatment in sand fill before discharge to native soil. Mounds are commonly used where there is high groundwater, shallow bedrock, or soils that are too slow or too fast.  More information can be found in the decentralized systems technology fact sheet on mound systems.

How Soil Tests Are Performed

Most evaluations use a backhoe to dig soil pits so the tester and, in many counties, an inspector can observe and hand-texture each horizon in the profile. They identify limiting factors and other characteristics that determine system type, depth, and square footage.

Mottling and Redoximorphic Features

Actual water does not need to be present to establish groundwater concerns. Mottling and redoximorphic features are color patterns created by periodic saturation and oxygen changes in soil. Their depth indicates the seasonal high water table for design purposes, even if no water is visible during the visit. More information can be found in the field indicators of hydric soils in the United States by the US department of agriculture.

Talk To A Certified Soil Tester

A Certified Soil Tester documents the site per SPS 385 and provides the report your designer and local authority need to approve a POWTS plan. If you have questions or want to schedule testing, call 262-968-2550 or use our contact form.